1. Field of the Invention
The present invention relates to a projection lens and more particularly to a projection lens with only a few lens elements. The projection lens has only minimal diameters, a short body and a long back focal distance. The aberrations of the projection lens are well corrected.
2. The Prior Art
There are various well-known types of projection lenses for projectors having three spatial light modulators or digital micromirror devices (DMD). In this kind of protector, the light produced falls on three matrices of micro-mirrors. The micro-mirrors are electronically guided and produce images which consist of three color channels. By means of a prism lens or beam concentrator, the images from the three color channels are combined to provide a single full-color image which is reproduced by the projection lens in enlarged form on a screen.
Because the beam concentrator can direct light to the projection lens only below a certain limiting angle, the projection lens has to be nearly telecentric in order to guarantee an optimal light source efficiency. In addition, the projection lens must have a high optical efficiency in order to produce a bright clear image.
In U.S. Pat. No. 5,914,818, such projection lenses are described. These lenses, however, have very large diameters and long assembly cases, and are for that reason heavy and expensive to manufacture.
In U.S. Pat. No. 5,625,495, a telecentric lens is shown. The lens, however, has too short a back-focal distance to use for the application described.
The projection lenses described in U.S. Pat. Nos. 5,969,876, 5,218,480 and 5,969,874 largely fail to meet the required condition of telecentricity.
In accordance with the invention, a lens is produced with small dimensions, high image quality, a small number of lenses, and of small dimensions. The projection lens is nearly telecentric and has a long back focal distance.
By conforming to certain prescribed lens combinations, the maximum angle of the principal ray to the optical axis on the modulator side of the lens amounts to two degrees or less, with the result that the projection lens is approximately telecentric.
Beginning with the object side of the lens, i.e. the side facing the projection screen, the projection lens comprises a positive lens element L1, a negative lens element L2, a positive lens element L3, or lens element group L3 Which in turn comprises a negative lens element L31 and a positive lens element L32, followed by a negative lens element L4, a positive lens element L5, and a positive lens element L6.